1. Field of the Invention
The embodiments of the invention generally relate to medical devices and assemblies, and more particularly to an orthopedic surgical implant assembly used in the field of surgical lumbar, thoracic, and cervical spine treatment.
2. Description of the Related Art
Surgical procedures treating spinal injuries are one of the most complex and challenging surgeries for both the patient and the surgeon. When there are various deformities, trauma, or fractures of the vertebra, surgeons may attempt to “fuse” them together by attaching screw-like devices into the pedicles of the spine and thereby connecting several vertebrae (typically two or more) using a semi-rigid rod. However, due to the complexity of the human anatomy, most surgeons must bend the rod (causing notches thereby reducing fatigue resistance) before placing them into two or more non-aligned pedicle screws that vary in height in order to properly stabilize the pedicle screw assembly within the patient's body. However, this bending causes notches and reduces fatigue resistance and wastes valuable surgery time before the surgeon is able to insert the rod. That is, the surgeon must sacrifice the freedom of optimal screw placement in the spine for ease of construct assembly.
Most conventional polyaxial screw systems generally consist of a bone screw with the top portion of that screw pivoting inside a screw head. This typical conventional design necessitates the bones screw to have a narrow neck just below the entrance to the bottom of the screw head. This allows clearance for the polyaxiality motion of the screw construct. However, this smaller and weaker neck portion is significantly further away from the forces being applied through the rod, which consequently allows a bigger moment arm and increases the chance of screw breakage at the weak neck portion.
Depending on the purpose of the spine surgery, indications, and patient size, surgeons must pre-operatively choose between different spinal systems with differing rod sizes pre-operatively sometimes causing delays in surgery while waiting for more adequate systems to be sterilized. Most conventional systems depend on deformation and notching of the rod to be able to lock it into the screw head. This tends to significantly reduce the fatigue life of the rod. Some surgeons prefer monoaxial screws for rigidity, while some sacrifice rigidity for surgical flexibility in screw placement. Therefore, a system is needed to accommodate both theories. For example, during scoliosis surgery conventional polyaxial systems typically cannot lock into a desired position to persuade the spinal column into the desired correction before final construct assembly.
Most conventional top loading polyaxial spine screws address cantilever failure by utilizing too much stress to the constructs making them weaker in other areas of concern. Moreover, most conventional polyaxial screws do not generally offer enough medial/lateral flexibility because the rod sits too closely on top of the center of rotation of the bone screw producing a smaller arc of rotation. Furthermore, most conventional titanium top loading screw systems only accommodate one rod size. Additionally, most conventional spinal implant designs can only accommodate either a monoaxial design or, separately, a polyaxial design, but not in one assembly. As such, most conventional screw assemblies cannot accommodate 3, 3.25, 3.5, and 4 mm rod sizes in one singular screw assembly. Typically, the particular size of rod used depends on the patient's size and other factors, which may not be determined until after the surgery begins and, potentially, only after the surgeon has already inserted the bone screw into the bone.
Generally, most conventional top loading polyaxial spine screws do not do enough to address cantilever failure of the assembly components. Additionally, most polyaxial screws generally do not offer enough flexibility because the rod sits too closely on top of the center of rotation of the bone screw producing a smaller arc of rotation. Furthermore, most conventional top loading screw systems generally do not accommodate different rod sizes. Moreover, most conventional polyaxial screws offer an equal degree of rotation or freedom referenced to the main screw axis. However, some portions of the spine do not need the system to provide equal polyaxial motion in all directions. For example, some portions of the spine require a range of 5 degrees in one direction and 45 degrees in the opposite direction on the same plane. Generally, most conventional systems simply provide 25 degrees all around. Thus, there remains a need for a new and improved pedicle screw assembly capable of overcoming the limitations of the conventional designs thereby providing the surgeon with improved intra-operative flexibility and the patient with an improved prognosis for better and complete rehabilitation.